Inner walls of the cylinder bores of internal combustion engines are subject to the wearing action of a piston and its seal structure. In engines fabricated from aluminum, aluminum alloys or other lightweight materials, cylinder liners are cast into the bores of the engine block to provide the necessary wear resistance. The casting of cast iron cylinder liners into aluminum blocks allows for excellent iron wearing surfaces in the bores of a lightweight fuel-efficient aluminum block engine.
One known process for inserting the cylinder liners into the engine block includes positioning the cylinder liner in the engine block mold prior to pouring molten metal into the mold cavity to form the engine block. After the casting is complete, these cast-in liners are permanently embedded within the cast metal walls of the cylinder bores.
Cylinder liners for use in the manufacture of engine blocks typically are manufactured using either conventional sand “static” casting or by centrifugal casting. While conventional sand casting incorporates insulating sand on the inside diameter and the outside diameter of the solidifying casting which helps to slow the solidification rate, the centrifugal casting technique does not have that advantage. Centrifugal casting uses a spinning permanent metallic mold within which the inside surface of the metal mold is coated with a thin coating of refractory materials and other processing fillers and/or additives. As a result of the limited insulating contribution of this coating, the high solidification rate of the surface of the centrifugal casting results in undesirable microstructures near the surface of the casting. Because the heat extraction rates are less severe from the inside to the outside of the solidifying casting, there will be a non-uniform microstructure of the iron from the outside to the inside of the casting. The depth to which the undesirable microstructure extends into the casting is a result of a variety of processing factors. This undesirable microstructure extending in from the surface of the casting can be very difficult to machine and will not exhibit the desired long wearing properties of the cylinder liner's bore surface for the operation of an engine. In addition, the coating material that is introduced into the spinning centrifugal mold becomes attached or embedded into the outside surface of the centrifugal casting. These impurities must then be removed from the outside of the centrifugal casting in a separate operation. In some cases such impurities can become sufficiently imbedded in the surface of the casting to cause the casting to be scrapped out.
When cast iron cylinder liners are used in aluminum block engines the effect of the differences of the coefficient of thermal expansion (CTF) of the two materials needs to be considered. Because the CTE's are different, under certain applications there may be separation of the outside surface of the cylinder liner from the aluminum block during the operation of the engine. The extent that this separation occurs and the consequences of it depend upon the design and type of the engine. To prevent such separation, in some applications mechanical bonding is specified between the outside of the cylinder liner and the aluminum block. In other applications, mechanical bonding is not specified. Many cylinder liners are cast into aluminum blocks that have no mechanical bonding at all between the liner and the block. While there is no mechanical bonding, it is still desirable to provide a continuous encapsulation (adherence) of the aluminum around the liner. To accomplish this adherence, the surface of the cylinder liner is roughened by appropriate machining threads and even pitting the surface using a process, such as shot blasting. Shot blasting, while roughening the surface, does not create cavities that provide casting material back drafts for mechanical locking or bonding. Shot blasting is accomplished using shot, which are substantially spherical particles with no angles or sharp edges. The outside surface subject to shot peening reduces the surface tension of the molten casting material during casting, allowing it to flow smoothly and completely around the entire surface of the liner. Shot peening of non-mechanically bonded cast-in cylinder liners is often used in applications such as water-cooled aluminum block engines, where mechanical bonding between the liner and block is typically not required.
However, in some applications utilizing cast iron cylinder liners in aluminum block engines, it is desirable or required to have mechanical bonding of the cast iron liner to the aluminum block. Such applications may include air-cooled aluminum block engines, as well as certain water-cooled aluminum block engines. Mechanical bonding is desirable for a number of reasons. For example, under certain conditions, such as extreme thermal cycling, a non-mechanically bonded liner may separate from the aluminum block because of the different CTE's of the two materials. A mechanical bond between the outside of the liner and the host aluminum block helps to ensure good thermal conductivity between the two materials and the bond can also add to the stability of the roundness of the liner in operation.
Two common approaches to accomplishing such mechanical bonding have been utilized with cast-in cast iron liners in aluminum block engines. One approach for static cast liners, as disclosed in U.S. Pat. No. 3,561,104 (the “'104 patent”), which is herein incorporated by reference in its entirety, involves casting vertical ribs on the outside which are subsequently peened over using centerless grinding during subsequent machining to provide a back draft condition and a resultant mechanical bond when cast into an aluminum block. Another technique used for centrifugally cast liners, as disclosed in U.S. Pat. No. 6,468,673 (the “'673 patent”), which is herein incorporated by reference in its entirety, involves casting in a manner to form a very rough exterior on the outside diameter of the centrifugal liner by utilizing an appropriate refractory coating on the inside of the spinning centrifugal mold. The rough as-cast exterior surface of the liner provides spines, mushrooms and crevices on the outside of the liner that provide a back draft condition and a mechanical bond when cast into an aluminum block.
While both the methods of the '104 patent and the '673 patent provide mechanical bonding between the cylinder liner and the aluminum block, they have some drawbacks. First, they both involve an as-cast outside diameter surface of the cylinder liner. The use of an as-cast surface brings with it the problems associated with cast iron microstructures near the surface of the casting. The surface of gray iron castings may include undesirable microstructures formed in the iron because of the faster solidification rates close to the surface of the casting. The surface effects, which are more pronounced in centrifugal casting compared to static casting, can extend well into the cross-section of the cylinder liner. If a thin wall cylinder liner is desired to be used with an as-cast outside diameter, such undesirable microstructure resulting from the surface effect may actually extend through the liner cross-section to the inside diameter wear surface of the very thin liner. The extension of the undesirable microstructure through the liner cross-section may result in both manufacturability problems and/or functional problems for the liner.
Another problem with the above-discussed methods includes the removal of the residual material from the outside of the liners. In centrifugal castings, the residual material is a refractory coating material, which had been sprayed onto the inside surface of the spinning centrifugal mold. In such centrifugal liners, the back drafted crevices formed by the mold coating process may actually trap the refractory coating material in the surface of the liner. In static casting, such as the '104 patent, there may be imbedded or loose sand grains in the vertical as-cast ribs on the outside diameter of the liner. Such residual impurities prevent the intimate contact between the iron and the aluminum, which is the purpose of the mechanical bonding. In addition, the capability of controlling dimensional tolerances on an as-cast surface (centrifugal or static cast liner) is limited due to casting variables. As-cast tolerances may not be tight enough to allow a minimum distance between multiple cylinder liners in a block and to provide the optimum uniformity of wall thickness.
What is needed is a cylinder liner and a method for making a cylinder liner that provides mechanical bonding with the engine block with clean and close toleranced surfaces, while maintaining substantially uniform and desired microstructure across the entire cross section of the cylinder liner. The current invention provides solutions to these problems that are associated with the use of cylinder liners with as-cast outside diameter surfaces.